Frequency divider and discriminator circuit



Aug. 19, 1947. CROSBY 2,425,923

FREQUENCY DIVIDER AND DISCRIMINATOR CIRCUIT Filed March '7, 1945 IfPzgaz/vcr 0/1405? 1 /6 .7

I In: #005 IN VEN TOR.

Maw/ y 6 (Poser A warm: y.

Patented Aug. 19, 1947 FREQUENCY DIVIDER AND DISCRIMINATOR CIRCUITMurray G. Crosby,

Riverhead, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application March 7, 1945, Serial No. 581,448

8 Claims.

My present invention relates generally to reception of angle modulatedcarrier waves, and more particularly to a combined frequency divider anddiscriminator circuit for frequency modulated (FM) carrier waves.

One of the important objects of my present invention is to provide an FMdiscriminatorrectifier network, wherein the discriminator circuits arenormally tuned to frequencies so widely spaced apart that the resonancecurves of the discriminator .circuits do not overlap at any pointsthereof, there being employed balanced rectifiers in conjunction withthe discriminator circuits to provide a Inodulatio-n-representativesignal.

Another important object of my invention is to provide a frequencydivider of the oscillating converter and amplifier type, 50 adjustedthat two tuned circuits thereof are off-tuned on opposite sides ofrespective carrier frequencies, whereby impression of an FM signal onthe tuned circuits causes the amplitude of the oscillations in the twotuned circuits to be modulated in opposite directions, in associationwith a differential detector network for detecting the amplitudemodulated oscillations.

Another object of my present invention is to provide a novel FMdetection system which eliminates the need for an amplitude limiter, andwhich may be adjusted automatically tosquelch noise in the absence ofreceived FM signals.

Still another object of my invention is to add further utility to afrequency divider network of the type disclosed and claimed in my U. S.Patent No. 2,344,678, granted March 21, 1944.

Still other objects of this invention are to improve generally theefiiciency and reliability of frequency divider networks in FMreceivers, and more especially to provide frequency division networksfunctioning to discriminate and to provide an output of substantiallyuniform amplitude without the use of an amplitude limiter.

Still other features of my invention will best be understood byreference to the following description, taken in connection with thedrawing, in which I have indicated diagrammatically a circuitorganization whereby my invention may be carried into effect. I

In the drawing:

Fig. 1 shows diagrammatically an embodiment of the invention;

Fig. 2 shows typical resonance curves of the discriminator circuits; and

Fig. 3 shows an alternative relation between the resonance curves of thediscriminator circuits.

and particularly No. 2,064,106, granted December 15, 1936, as well as inmy U. S. Patent No. 2,230,231, granted February 4, 1941. In the circuitsof these patents, prior to demodulation of the frequency modulatedcarrier waves the latter are subjected to frequency division in such amanner that the frequency swing of the modulated carrier wave is reducedin [the same proportion as the center frequency of the wave.

It will be understood that the frequency divider shown in Fig. 1 may beconsidered, for a specific utilization, as being located following theintermediate frequency (I. F.) amplifier network of an FM receiver ofthe superheterodyne type. It is, however, to be clearly understood thatthe present invention is not limited to a superheterodyne receiver, norto the specific frequency values given, nor to the specific frequencymodulation. The generic expression angle modulated is intended to coverboth phase modulation, frequency modulation and hybrid modulations whichpossess characteristics common to :both phase and frequency modulation.

For the purpose of the present application, let it be assumed that thesuperheterodyne receiver is one employed in the present FM broadcastband of 40-50 megacycles (mc.), and that the first detector or converterreduces the center or carrier frequency of the modulated carrier wavesto an I. F. value chosen from a range of 2 to 20 me. In such caseadvantages are secured if a frequency division network is utilized toreduce the center frequency, say 4 mc., by a desired factor which is aninteger. As explained in my aforementioned patents, the frequency swingof the I. F. energy is simultaneously decreased by the same factor. If,for example, the center frequency is reduced by a factor of 4, then thefrequency swing would be reduced by the same factor of 4. Since, inaccordance with present standards of FM broadcasting, the maximumpermissible frequency deviation in use in the 40-50 mc. band iskilocycles (kc.) to either side of the center frequency, it will be seenthat reducing the center frequency by a factor of 4 transmitted throughwill result in a reduction of the maximum overall frequency swing from150 kc. to 37.5 kc.

Specifically considering the circuit of Fig, 1, let it be assumed thatnumeral I designates the usual intermediat frequency transformer coupledto the plate circuit of the last intermediate frequency, amplifier tube.In that-case .each .of the primary and secondary resonant circuits 2 and3 would be tuned to the center frequency F. As stated before, and merelyby way of specific illustration, F is assumed to have a value of 4 mc.As is well known, the center or carrier frequency is deviated at thetransmitter in dependence upon the amplitude of the modulation signal,and'the rate of overall frequency deviation isdependent.

upon the modulating frequencies themselves. The band width oftransformer. I will be in excess of the maximum frequency swing and willin the case assumed preferably be approximately 200 kc.

The-numeral 4 designates the converter tube which may be of the-6SA'7type, althoughthe inyentionisnot restricted to this type-of tube nor toa; tube of the pentagridtype. Generally, the

tube may comprise a-cathode 5 and an output .electrode, or plate, 6.Between the cathode 5 and plate. 6 are arrangedin sequential relation asignal input grid. Lapositive screen grid 8, a secnd, signal, oroscillation, input electrode 9, a

- positive screen grid I0 and the usual suppressor grid II.

1 The cathode is connected to ground I through the customary biasingresistor I2 shuntedban intermediate frequency b -pass condenser. -'Ihe-1ow potential side of secondary circuitf3 .issestablished at groundpotential, while theopposite s de is connected to the signal grid .7..Theelectrode 9.is connected through resistor i3 to ,the grounded-end ofbiasing resistor I2. The functions of grids 'I .and .9 may beinterchangedif desired.

,The plate-,6 is connected to the positive terminal +13 of a directcurrent source: through a resonant =output circuit comprising a coil I4shunted by'condenser I5. The circuit I4, I5 is presentlyassumed tobetuned to a frequency which is an integer. Thesignalmodulated car rier YQ.tage, develop,ed across circuit I4, I5 is the condenser. I6. which hasa low impedance to the carrier voltage. As heretofore explained, itis,desired tojdivide the center frequency'of themodulated carrier wavesand se-- curesimultaneousproportional reduction of the frequency swing,whichv is. taken as twice the fr quency deviation. ,Hence, the effectivefrequencyswing-of the energy in circuit I4, I5 would be, reduceduntimes. 'If. the effective band passwidth ,at 1 transformer .I .was 200kc., .and .n equalled'4 then. thepass band widthat .tuned circuit I4,,I5 ,would be.approximately,.50 kc.

,The frequency multiplier tube is designated by numeral 2B,,and it may.bea tube of the pentode' type suchas ,is.,now known commercially as theGSKI. The, cathode 2| ofthis tube includes a self-biasingresistor- 22connected to ground-and shunted by anappropriate .bypass condenser.

"The input grid}23 .isconnected-by the coupling condenser 24 to the,highpotential side .of'circuit I4, I5, the grid being connected togroundthrough the: leak resistor,- 25.

is-connectedby lead.,3I and coupling condenser :32 to the grid. 9.,The'plate 30 is, also, connected.

/ presently assumed to be equal to F (Hi1);

In the specific illustration given, the resonant frequency of circuit40, 4I would be either 3 mo.

or 5 mc. The biasing resistor 22 is given a value such that, in theabsence of input voltage to grid 23, directcurrent voltage developedacross resistor 22 is. sufficient to bias grid 23 close to plate currentcut-off. It will now be realized that the resonant frequency voltagedeveloped across the circuit .40, 4| is impressed upon the electrode 9simultaneously with the impression of the frequency-modulated carrierwave energy of center r frequency F upon grid I.

The frequency modulated carrier wave voltages impressed on respectivegrids I; and 9 are heterodyned by virtue of electron coupling, andproduce the beatfrequency. voltage Whose center. frequency is plicationofnil times, and this multiplied output is. then. fed to the electrode 9.of the converter tube 4 so that the output of the converter consists ofthe heterodyne beat between the center frequency R and the frequenc ofthe voltage developedacross circuits, M. It will be seen that underthese circumstances an oscillationwill exist in circuit I4, I5 when theconverter gain and the multiplier gain exceedunity. The converter gaindepends upon the strength of the signal applied from the inputtransformer I. Hence, in

the absence of signal energy-at the input transformer I there-will be nooutput voltage at circuit I4, I5- since there=will be no oscillationsproduced through the multiplier tube. Tubes 4 and -2G cooperate with theassociated tuned circuits to provide a re-entrant circuit i.-e.-, aregenerative modulation-circuit, The constants of the tubes 4 and 20arechosen-so'thatwhen the signal inputlevel exceeds a predeterminedamplitude'then the re-entrant action becomes sufficient to sustainoscillation.

. Additionally, the network-may have its constants adjusted so when thesignal input voltage is raised. above the level required to startoscillations the-output voltage-does not appreciably .increase.Accordingly,-when used todivide the frequency,- -or phase, deviation ina frequency, or

phase, -modulat-ion receiver, the division'network .also. permits theusual limiter stage employed rior. to the detector tobe dispensed with.

A further advantage of this type'of frequency divider, when employed ina frequency, or, phase,

modulation receiver, is-in the selective ,action thereof. Due to theresonant frequenciesbf the circuits employed in 1 the re enerationcircuit,

there is a limited range-19f frequency-F whichwill cause the systemto'oscillate and produce an out- ;put' voltage. "Thus; the receiver-maybe tuned to the proper center frequency F so as to produce oscillationsin the divider, and interfering signals which do not have the frequencyF, but which come through the usual selective circuits, will berejected.

A still further advantage which this division circuit has for use infrequency, or phase, modulation receivers is its threshold action. Thelatter can be utilized to eliminate noise from the receiver in theabsence of signals. The usual frequency modulation receiver produces aroar of noise when there is no signal present. This is due to the factthat the limiter increases the gain of the receiver so as to amplify thenoise to full volume. However, with the present type of frequencydivider network, the gain of the receiver may be adjusted so that thethreshold of the divider is just above the noise level. Hence, therewill be no output voltage unless a signal is present which is strongerthan, and overrides, the noise level. It will therefore, be appreciatedthat the present frequency division network improves the signal to noiseratio at the demodulator input circuit by virtue of the conjoint actionof the division of the frequency swing of the frequency modulatedcarrier waves and the threshold action of the network itself.Furthermore, by virtue of the improved selectivity and the substantiallconstant output, distortion is greatl minimized.

In accordance with my present invention I utilize both tuned circuitsHi, l5 and 40, 4| as discriminator input circuits. Instead of tuningthese circuits to the respective frequencies n and I tune themrespectively to frequencies below and above said frequencies or viceversa. Assuming the specific illustrative examples given above, circuitM, l5 may be tuned to a frequency less than 1 me. by a predeterminedamount, and circuit 10, 4| is in such case tuned to a frequency greaterthan 5 me. (or 3 Inc.) by a predetermined amount. As a result of suchrelatively opposite detuning of circuits l4, l5 and 43, 4| there isproduced amplitude modulated carrier wave energy across each of thecircuits.

Rectifiers 42 and H are shown as being of the diode type; it is to beunderstood that any other well known form of detector device may beutilized in place of the diode rectifiers. Anode 43 of diode 42 iscoupled to the plate side of circuit l4, l5 through condenser l6.Cathode 44 of diode 42 is connected to ground through load resistors 46and 50 arranged in series. I. F, bypass condensers t7 and 5| shuntrespective resistors 46 and 5D. Resistor 45 provides a direct currentreturn path from anode 43 to the negative end of resistor 46. Resistor52 is also a direct current return path and connects cathode 49 of diode4| to ground. An I. F. choke of high impedance at the frequency ofcircuit 40, 4| may replace resistor 52. Condenser it? couples thecathode end of resistor 52 to the plate side of circuit fill, 6|. Itwill be obvious that condensers l5 and 46 function as direct currentblocking condensers.

The modulation-representative voltage is taken off between the cathodeend of resistor 46 and ground by leads 53. In other words, the rectifiedsignal voltages across resistors 45 and 59 are combined in differentialor polarity opposing relation so as to provide the usual advantages of abalanced FM detector. The leads 53 may feed the modulation signalvoltage to any desired and suitable amplifier which may be followed byan ultimate reproducer.

The functioning of the detector circuits will now be described, it beingpointed out that Figs. 2 and 3 show curves illustrative of theinvention. In Fig. 2 I have shown the relation between the amplitude vs.frequency curves of circuits |4, I5 and 40, 4| for the condition wherethe normal frequency of the latter would be while in Fig. 3 the curvesare shown for the case where circuit 4|), M has a normal frequency of InFig. 2 the input signal frequency is shown located by the verticaldotted line F- at 4 me. The output circuit I4, I5 is tuned, inaccordance with my invention, to a frequency sufficiently less than 1me. to permit the frequency of 1 me. to fall on a substantially linearportion of the upper flank of resonance curve a. In other words, ifcurve 0. represents the resonance curve of circuit i i, I5 for theoff-tune condition, the value may be indicated to fall at point I) ofresonance curve a. The dotted line curve 0 shows what the resonancecurve of circuit I l, l5 would be if it were tuned to the frequency Inthe same way curve d illustrates the resonance curve of circuit Gil, 4|,and shows circuit 40, 4| to be tuned to a frequency somewhat higher thanthe frequency Dotted line curve e shows what the resonance curve ofcircuit 48, 4| would be if it were tuned to 5 me. The frequency or 5mc., falls at point g 0f the lower flank of resonance curve 01. Here,again, point g should fall at a substantially linear portion of thelower flank of resonance curve :2.

It is important to locate points b and g on respective curves at and ifso that the maximum frequency swings at circuits I 3, I 5 and Mi, 4| canbe accommodated. The maximum frequency swing at input circuit i has beenassumed to be kc. Under these conditions the maximum frequency swing atcircuit |4, H5 is 150 kc. divided by 4, which is 37.5 kc. Hence, point bshould be located so that the maximum frequency swing of the I. F.energy at circuit l4, l5 will be 18.75 kc., above and 18.75 kc., belowpoint 1) without going off the linear part of the resonance curve. Atcircuit 46, 4| the maximum frequency swing is 37.5 kc. multiplied by 5,or 187.5 kc. Therefore, point 9 is located on the resonance curve at,and the resonance curve at is chosen accordingly, so that the maximumfrequency swing of the energyatizcircuit 40,f4|: will be.93.7 5.

The frequency F of. the inputsignals falls above the peak frequency ofcircuit 40, 4| in that case. However, the point g' on curve at will belocated as explained in connection with curve (1. An examination ofpoints b and g (or g) reveals that operation along these points isproductive of variable-amplitude carrier energy at respective circuits|4, l and 40-, 4|.

Itcan be seen from Figs. 2 and 3 that when the signal input energyinstantaneously deviates higher in frequency, that is shifts above thevalue F, the energ developed at circuit |4, I5 is modulated'towards alower amplitude while the energy developed at circuit 40; ll-isconcurrently modulated towards a higher amplitude. The reverse is; ofcourse, true as the instantaneous signal input frequency deviates to afrequency less than F. In other words, Figs. 2 and 3 clearly show thatas the instantaneous frequency of the signal input energy deviates belowand above frequency F the amplitudes of signal energy at circuits l4, I5and 40, 4| will correspondingly vary, but in opposite senses.

This is the familiar condition required for operation of a balanced FMdetector. By rectifying the variable amplitudesignal energy at each ofcircuits |4, I5 and 40, 4| by means of respective rectifiers 42 and 4|,there is provided rectified signal voltage across each of load resistors46 and 5B. These output resistors 46 and 50 are connected so as toprovide a differential modulation signal voltage which is representativeof the amplitude modulations superimposed on the FM waves in theseparate tuned circuits |4, |5- and 40, ll. Although in the foregoingdescription, the circuit |4, |5 has been described as detuned to afrequency below and circuit 40, 6| has been described as detuned to afrequency above as the case may be, this action may be reversed in thatcircuit M, l5 may be detuned to a frequency above and circuit 40', 4 I-may be detuned to a frequency below ZOtil) as will be apparent to thoseskilled in the art.

By a proper adjustment of the grid leak resistors and electrode voltagesof tubes 4 and 26, the amplitude of the oscillations in the re-entrantcircuit may be mad independent of the amplitude of the FM signal. over arange of signal inputs. This characteristic tends'to give oscillatoroutput of uniform amplitude despite amplitude modulation on the incomingFM signal, and, therefore, the circuit does not require anamplitudelimiter. The system, of course, possesses the various othercharacteristics described above and claimed in my aforesaid U. S. PatentNo. 2,344,678; It will beparticularly noted that although circuits l4,l5 and 40; 4| function as discriminator circuits, yet the resonancecurves thereof do not overlap at any point and are in fact relativelywidely spaced in the frequency spectrum.

WhileI have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization. shownand described, but that many modifications may be made without departingfrom the scope of m invention.

What I claim is:

1. In combination with a pair of rectifiers having, respective loadresistors and a differential output voltage circuit common to saidresistors, a first tuned circuit connected to one rectifier, a secondtuned circuit connected to the second rectifier, a source of anglemodulated carrier waves having a predetermined center frequency, saidfirst tuned circuit being resonant to a first frequency less than apredetermined subharmonic frequency of said center frequency, saidsubharmonic'frequency falling onthe upper flank of the resonance curveof the first tuned circuit, said second tuned circuit being resonant toa second frequency in excess of a third frequency which differs from thesource center frequency by said subharmonic frequency, said thirdfrequency fallingon the lower flank of the resonance curve of the secondtuned circuit, frequency multiplying means coupling said first tunedcircuit to said second tuned circuit, and means responsive to saidsource waves and frequency multiplied wave energy for developing in saidfirst tuned circuit angle modulated waves whose center frequency isreduced to said subharmonic frequency value.

2. In a frequency modulation discriminator of the type comprising a pairof differentially connected rectifiers; the improvement comprisingrespective resonant input circuits for said rectifiers, a source offrequency modulated waves tuned to a predetermined frequency, one of theinput circuits being tuned to a predetermined frequency difference froma desired subharmonic frequency of the source frequency, saidsubharmonic frequency being located on a flank of the resonance curve ofsaid one input circuit, the second input circuit'being tuned to apredetermined frequency difference in an opposite sense from a frequencymultiple of the subharmonic frequency, said frequency multiple beinglocated on the opposite flank of the resonance curve of the second tunedinput circuit, and connections including said source and two inputcircuits in a closed reentrant system.

3. In combination with a pair of diode rectifiers having respective loadresistors and a differential output circuit common to said resistors, afirst tuned circuit connected to one rectifier, a second tuned circuitconnected to the second rectifier, a source of frequency modulatedcarrier waves having a predetermined carrier frequency, said first tunedcircuit being resonant to a frequency less than a predeterminedsubharmonic frequency of said carrier frequency,- said subharmonicfrequency being located on the upper flank of the resonance curve of thefirst tuned circuit, said second tuned circuit being resonant to asecond frequency lIleXCBSS of a third frequency which differs from saidcarrier frequency by said subharmonic frequency, said third frequencybeing located on the lower flank of the resonance curve of the secondtuned circuit, a frequency multiplier coupling the first tuned circuitto said second tuned circuit, and means for developing in said firsttuned circuit frequency modulated waves whose carrier frequency isreduced to said subharmonic frequency value,

4. In combination with a pair of differentially connected rectifiers,respective resonant input circuits for said rectifiers, a source ofangle modulated carrier waves tuned to a predetermined carrierfrequency, one of the input circuits being tuned a predeterminedfrequency difference from a desired subharmonic frequency of saidcarrier frequency, said subharmonic frequency being located on one flankof theresonance curve of said one input circuit, the second inputcircuit being tuned a predetermined frequency difference in an oppositesense from a frequency multiple of the subharmonic frequency, saidfrequency multiple falling on the opposite flank of the resonance curveof the second input circuit, and connections including said source andtwo input circuits in a closed system.

5. In a detector of frequency modulated carrier waves, a pair ofoppositely connected diode rectifiers, respective resonant inputcircuits for the rectifiers, said input circuits being tuned to resonantfrequencies widely separated to prevent overlapping of the resonancecurves thereof, and means, responsive to said waves, for applying tosaid input circuits modulated waves whose center frequencies aresufficiently different from the said separated resonant frequencies tocause ampli tude-variable wave energy to appear in each rectifier inputcircuit and the center frequency of the waves applied to each of saidinput circuits being located on respectively opposite flanks of saidresonance curves.

6. In combination with a, pair of detectors having respective loadresistors and differential output voltage circuit common to saidresistors, a first tuned circuit connected to one rectifier, a secondtuned circuit connected to the second rectifier, a source of anglemodulated carrier waves having a predetermined center frequency, afrequency divider having said first tuned circuit as its output circuit,said first tuned circuit being resonant to a frequency less than apredetermined subharmonic frequency of said center frequency, saidsubharmonic frequency fallingon the upper flank of the resonance curveof said first tuned circuit, a frequency multiplier including saidsecond tuned 10 lated waves whose center frequency is reduced to saidsubharmonic frequency value.

'7. In a frequency modulation discriminator of the type comprising apair of differentially connected rectifiers; the improvement comprisingre- 15 spective resonant input circuits for said rectifiers,

a source of frequency modulated waves tuned to a predeterminedfrequency, a frequency divider including one of the input circuits tuneda predetermined frequency difference from a desired subharmonicfrequency of the source frequency, said subharmonic frequency beinglocated on one flank of the resonance curve of said one input circuit, afrequency multiplier including the second input circuit tuned apredetermined frequency difference in an opposite sense from thefrequency multiple of the subharmonic frequency, said frequency multiplebeing located on the opposite flank of the resonance curve of the secondinput circuit, and connections including said source and two inputcircuits in a closed re-entrant system.

8. In combination, a pair of difierentially connected detectors,respective input circuits for the detectors, said input circuits beingtuned to spaced resonant frequencies to prevent overlapping of theresonance curves thereof, and means, responsive to angle modulatedwaves, for applying to said input circuits angle modulated waves whosecen ter frequencies are substantially different from said spacedresonant frequencies and the center frequencies of the waves applied tothe respective input circuits being located on respectively 013- positeflanks of said resonance curves.

MURRAY G. CROSBY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

